Proteins optical configuration

In every living cell, proteins are assembled on ribosomes by the co-polymerization of about 20 kinds of amino acids, all with the same L-optical configuration (see Fig. 5.7). The molecular weight of proteins varies from about 6000 to well over a million, but all have the common structure shown in Fig. 4.2, where R represents the familiar side-chains, such as methyl (for alanine) and j -hydroxy-benzyl (for tyrosine). The Atlas of Protein Sequence and Structure (National Biomedical Research Foundation, Georgetown University, Washington, D.C.), published at intervals, contains the amino acid sequences of many hundreds of proteins (for other lists, see Section 17.4). 

Let us consider a stochastic system described by a generic variable C. This variable may stand for the position of a bead in an optical trap, the velocity field of a fluid, the current passing through a resistance, of the number of native contacts in a protein. A trajectory or path V in configurational space is described by a discrete sequence of configurations in phase space. 

The a-carbon of each amino acid is attached to four different chemi cal groups and is, therefore, a chiral or optically active carbon atom. Glycine is the exception because its a-carbon has two hydro gen substituents and, therefore, is optically inactive. [Note Amino acids that have an asymmetric center at the a-carbon can exist in two forms, designated D and L, that are mirror images of each other (Figure 1.8). The two forms in each pair are termed stereoisomers, optical isomers, or enantiomers.] All amino acids found in proteins are of the L-configuration. However, D-amino acids are found in some antibiotics and in bacterial cell walls. (See p. 250 for a discus sion of D-amino acid metabolism.)... 

The main general routes for the synthesis of the a-amino acids are illustrated in the following sections (pp. 746-750). Some of the natural amino acids can be isolated from the hydrolsates of suitable proteins examples are provided on p. 750. They are optically active having the l [(S)] configuration. The synthetic products, formed by routes not involving asymmetric syntheses, are of of course racemic, from which the optically active forms may be obtained by the process of resolution. Some resolution procedures are discussed in Section 5.19. 

Abstract Now an incisive probe of biomolecular structure, Raman optical activity (ROA) measures a small difference in Raman scattering from chiral molecules in right- and left-circularly polarized light. As ROA spectra measure vibrational optical activity, they contain highly informative band structures sensitive to the secondary and tertiary structures of proteins, nucleic acids, viruses and carbohydrates as well as the absolute configurations of small molecules. In this review we present a survey of recent studies on biomolecular structure and dynamics using ROA and also a discussion of future applications of this powerful new technique in biomedical research. 

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